There have
been a number of articles in Audio Amateur, Glass
Audio, Electronics (nee Wireless) World etc.
that describe tube and transistor curve tracers that
would satisfy one or more of my interests in
examining the characteristic curves of vacuum tubes
and semiconductor devices. Nevertheless, none of
the designs are flexible enough the accommodate the
potential uses which a microcontroller,
analog-to-digital converter and PC can be used for.
This article describes a modular,
microprocessor-based curve tracer, based upon the
Parallax Inc. Basic Stamp BS2 with a design that is
sufficiently flexible to allow the unit to plot and
display several variables in real time. In
addition, the problem of keeping high voltages off a
computer motherboard is addressed with the use of
readily available opto-isolators. The Basic Stamp II
was chosen since there is a large community of users
for the device, a number of good books written on
the devices, and the ease with which it is
programmed. The prototype MCTracer is shown in the
photograph below:

The MCTracer
is designed so that two devices can be measured and
matched. Thus the two triode sections of 12AU7’s,
or a pair of 6L6’s (or with a little ingenuity a
quad) can be quickly compared.

Tube and Transistor Tracers:

Most
tube and transistor curve tracers use a ramp
generator to provide the voltages for plate and
grid, gate and drain etc. As the drive voltage is
swept, an oscilloscope display is triggered (or the
X-Axis is driven) and displays the results.
Borberly described a tracer in Audio Amateur
in 1990[1]
and provided a list of references to such designs.
Petrowsky offered a clever vacuum curve tracer using
the X – Y axes inputs of an oscilloscope[2].
While Petrowsky’s Tube Tracer was useful in helping
determine the merits of a particular bottle, it
would not allow the user to store and record data,
or to compare data among lots of tubes for
matching. Although Joe Carr did not offer a
specific tracer design in Ham Radio[3]
he did lay out the fundamentals for power tube
testing and provided the schematic for a gm
tester for high power transmitting tubes.

The
MCTracer, as shown here, departs from the strictly
analog approach by using an inexpensive and easily
programmed chip, the Basic Stamp II from Parallax[4]
to monitor the output from an Analog to Digital
Converter and feed the readings to the serial port
of a personal computer. A six input, 10 bit analog
to digital converter (Linear Tech LTC1093) and my
trusty (but modified) thirty-year old Heathkit IP-17
adjustable power supply (or other similar adjustable
supply such as the Eico 1030) are used to read and
provide the plate, grid and filament voltages to the
device under test. Instead of a ramp generator,
the “Armstrong Method” (i.e. using your arm) is
employed. The user sets a grid voltage (C-) and
simply turns the knob controlling B+ to generate a
plot of plate current (Y-axis) as a function of
plate voltage (X-axis) while the microprocessor
collects data and transmits it to the PC. Multiple
plots can be obtained by pausing the microprocessor,
setting a new grid voltage, and again sweeping the
plate voltage by hand. (In Part II we describe a
Digital to Analog Converter and Voltage Amplifier
that automates the entire process.) Timing circuits
and a ramp generator are unnecessary as the
free-running analog to digital converter repeatedly
transmits data to the PC at 15 millisecond intervals
(66 samples per second) using a recently published,
Microsoft ExcelÔ
Macro, StampDAQ for Parallax Inc. and available for
free on the Parallax website (www.parallaxinc.com
)

Circuit
Description:

The
circuit consists of an op-amp front end with
adjustable voltage dividers and a Linear Technology
LTC1093 10 bit, 6 input successive approximation
analog to digital converter. The LTC1093 is capable
of bipolar measurements by setting LOW the Unipolar
bit of its instruction set. This eases the design
and provides flexibility in measurements. The
LTC1093 requires careful attention to detail in
placing the ground planes since the device uses a
10-bit capacitative DAC. As pictured above, I used
copper foil to place a ground plane on the top of
the PCBoard as well as the bottom. The inputs to the
ADC are filtered with a small RC network to reduce
noise.

Linear
Tech LT1013 and 1014 opamps are specified in this
design because of their low leakage current, high
accuracy, and a modest degree of protection provided
by their input circuitry. Other high quality opamps
could be used in their place. (Note, Texas
Instruments now provides a direct substitute for
these op-amps.) The high voltage resistive voltage
dividers employ 3 ½ watt resistors stacked together
to avoid the problems of non-linearity at high
voltages. Importantly, two power supplies are used
so that the analog opto-isolators keep high voltages
from straying into the circuitry of the MCTracer,
and potentially onto the computer motherboard (with
disastrous results!). High voltages can creep across
a circuit board due to grease, dust, fingerprints,
poor soldering techniques and a variety of other
reasons. Thus, I used an epoxy coating for the area
of the circuit board where high voltages might be
present, particularly around and underneath the
input headers.

The first
power supply for the opamps, ADC and voltage
reference derives its current from the 12.6VAC
filament windings of the high voltage supply, is
pre-regulated with a 7815 voltage regulator, and
split into positive and negative 5.6 volt rails with
an artificial ground using LM317LZ and LM337LZ low
power voltage regulators. Power for microcomputer
and the isolated side of the opto-isolators is
obtained from a wall-wart transformer and regulated
with an

The
schematic diagrams for the various components of the
MCTracer are illustrated below:

The basic
amplifier unit consists of a non-inverting amplifier
with a gain of 2.0 and a resistive voltage divider.
The divider is adjusted with a 15 turn potentiometer
so that the maximum non-inverting voltage is equal
to 2.5 Volts. R(a) for the high voltage measurement
circuits should be made up from three 270K resistors
to minimize the non-linearities of carbon resistors
at high voltage. Dual supplies were chosen to make
the unit “modular” in that positive and negative
voltages could be measured and digitized. The
voltage range and values for R(a) and R(b) for each
of the 6 amplifier stages are given in the table
below.